Histological localization of binding sites of alpha-bungarotoxin and of antibodies specific to acetylcholine receptor in goldfish optic nerve and tectum.

Goldfish optic nerve as well as ganglion cell neurites grown in culture selectively bind rhodamine-labeled alpha-bungarotoxin following tissue fixation. Binding is competed for by unlabeled bungarotoxin, by carbamylcholine and tubocurarine, but not by atropine. In cross-sections, the label is seen confined to axonal bundles. The binding is not detectable without prior fixation and is very faint in brain sections, even after fixation. To further establish the nature of the binding, immunocytochemical studies were performed, taking advantage of a high cross-reactivity found between goldfish brain and antibodies against eel acetylcholine receptor (AChR). Antigenic sites were detected by an indirect unlabeled antibody complexed to horseradish peroxidase. Anti-AChR antibody binding to optic nerve and neurites in culture correlated with that seen with alpha-bungarotoxin. Binding of anti-AChR was observed in the brain, and was reduced in the denervated tectum following unilateral optic nerve crush or enucleation. The results are discussed in relation to functions of receptor proteins in the retinotectal system.